The technology development is proceeding very fast in the field of information-recording/-reproducing device using a magnetic disc or the like information recording medium (hereinafter which device is referred to also as disc device). As a result, the disc devices are finding many new application fields, besides the conventional computer-related uses, and various portable electronic appliances, such as the cellular phones, PDAs (Personal Digital Assistances), digital cameras, for example, are familiar in our day to day life, and are getting an increasing popularity. Such equipment is in need of a disc device that has a larger storage capacity and a high-speed access capability.
In a conventional disc device, there are a spindle motor for driving and a disc-shape information-recording medium such as a magnetic disc or an optical disc (hereinafter simply referred to as recording medium) fixed on a turntable of the spindle motor. The disc is put into rotation at a certain predetermined rotating speed, and head records and reproduces information on or from the recording medium magnetically or optically. FIG. 32 shows the structure in substantial portion of a hard disc drive, or a disc device using a magnetic disc for the recording medium.
FIG. 32(a) is a plan view, FIG. 32(b) is a cross sectional view of the P O P′ part of a disc device 801. A spindle motor for rotating a recording medium 802 and an actuator 817 for actuating a magnetic head are mounted and fixed in a case 818. It is hermetically sealed by a cover 819 to avoid dusts from the outside and prevent occurrence of turbulence of the air within the case. A magnetic head arm 816 is supported at one end by a bearing to be freely rotatable, and driven by a magnetic head actuator 817 so that a magnetic head slider 815 attached to the magnetic head arm 816 moves to a certain specific track of a recording medium 802. Recording/reproducing of information to and from the recording medium 802 is performed by a magnetic head (not shown), or an optical pickup (not shown) having an object lens for collecting light, through a known procedure.
The recording medium 802 is formed of an axle portion consisting of a rotating axle (spindle) 812 and a hub 811, and a disc-shape substrate having a recording layer of magnetic material provided on the surface thereof connected fixed together. The axle portion is supported to be freely rotatable by a hydrodynamic bearing mounted on a base 803 of the spindle motor at the center of a round hollow, which hydrodynamic bearing consisting of the rotating axle 812, a bearing sleeve 809 and a thrust support plate 810. The rotating axle 812 is coupled with the hub 811 press-fit or glued thereto. The round recording medium 802 is placed on a platform provided by a protrusion extending from the outer circumference of hub 811, and fixed to the rotating axle 812 by means of a round pressing plate 813 and a screw 814. Provided underneath the platform of hub 811, on which the recording medium 802 is disposed, are a ring-shaped rotor yoke 804 and a ring-shaped rotating magnet 805 magnetized in plurality of magnetic poles disposed along the circumference in the central portion. On the other hand, a stator 808 formed of an iron core 806 and a coil 807 is mounted fixed on the base 803 along the inner circumference of the central round hollow so that it opposes to the rotating magnet 805. These complete a spindle motor for rotating the recording medium 802 integrally formed of an axle portion and a recording portion.
Besides the above described inner rotor type motor, an outer rotor type motor may be used for the driving motor; where, a ring-shaped rotor yoke and a ring-shaped rotating magnet magnetized in plurality of magnetic poles are disposed along the outer circumference of a recording medium while a stator formed of iron core and coil is fixed to an axle support member provided in the central part of base. The driving means can be provided in many more variations; a structure where a stator rotates may be considered. As to the bearing of axle, a ball bearing, a metal bearing, etc. can be used instead besides the hydrodynamic bearing. As described in the foregoing, many of the spindle motors of the disc device are trying to make themselves thinner or slimmer by providing a hub in the vicinity of a rotating axle (spindle) for fixing the substrate of recording medium, and providing the rotor magnet thereon, and a stator at the periphery of the hub.
Among the information-recording/-reproducing devices (also called as disc device), the disc portion, on which a disc-form recording medium is mounted, is strongly requested to be smaller in the diameter and thinner in the overall thickness so that the device can be incorporated in a portable apparatus. Operating environments of portable apparatus, however, are much harsher than those of personal computers. Therefore, it needs to be designed so that it can withstand a substantial impact to be given when, for example, it is dropped. The downsized and thinner disc portions are expected to be rigid enough to withstand the shock.
In a case when a disc device is dropped on the ground, the impact acceleration easily reach several thousand times that the normal gravity acceleration. In the disc devices, a gimbaled head assembly with some spring property supports most the heads disposed facing to the flat surface of disc recording medium. So, in the day-to-day operating environments the head can chase the disc and keeps on its operation, even if the disc surface is somewhat deviated out of the rotating plane to a different disc height. However, even if an instantaneous impact caused the rotating portion to be withdrawn from stationary supporting means for a substantial amount, the head supporting mechanism as well as the recording medium itself might be destroyed. In a case of a hard disc drive using a magnetic disc, for example, the amount of dislocation allowable for a rotating body is approximately 0.2 mm at the most. If a rotating body is withdrawn substantially, a lubricant provided in a gap between the axle and bearing sleeve spills out and the bearing becomes unable to operate. Furthermore, the sputtering lubricant may well stain the disc portion. Thus, the amount of dislocation should be suppressed to be as small as possible.
There is a ramp load mechanism, which supports the head apart from the recording medium when head actuator moved in off-duty zone. This is aimed to prevent the head from colliding with magnetic disc at a shock. Several practical means have been proposed for the ramp load mechanism; some make use of outer circumferential edge portion, others make use of inner circumference portion of a disc out of the information storage region.
It has been necessary among the conventional hard disc drives and the like disc devices, where information is recorded in and reproduced from a recording medium using a magnetic technology, to take appropriate countermeasures to suppress influence of the magnetic fields escaping from spindle motor or other magnetic members to be effected on the recording medium. Some technologies have been proposed for avoiding the influence of escaping magnetic fields generated during operation of a disc device; which include provision of a shielding member against the magnetic fields.
In the above-described structure, where the rotating axle 812 is attached to the hub 811 and the recording medium 802 is supported by a platform provided by an extrusion from the outer circumference of the hub 811, it is difficult to connect the platform of hub 811 to the rotating axle 812 so that the face of platform for placing a recording medium, or the recording surface of recording medium 802, at a precise right angle with the axis of rotating axle 812. It is also difficult to bring the centers of flange portion and recording medium 802 to be concentric to the axis of rotation.
If there is an error in the angle formed by the platform of hub 811 and the rotating axis of axle 812, or that formed by the recording surface of recording medium 802 and the rotating axis of axle 812, the recording medium 802 revolves with a tilt on the recording surface. This results in a deviation in the position of recording surface of the recording medium 802, or a deviation in the surface. If there is a dislocation in the center of platform of hub 811 or the center of recording medium 802 from the center of rotation, the rotating recording medium 802 shows a dislocation in the direction parallel to the surface, or a deviation in the rotating axis.
In practice, the deviation in the surface and the deviation in the rotating axis appear integrated together. So, a recording density of a recording medium 802 is subject to these factors, which means that there is a limit in the efforts for increasing the recording density of a recording medium.
In order to suppress the deviations in the surface and in the rotating axis with a rotating recording medium 802, the accuracy in parts machining and in parts assembly have to be raised. Improving the accuracy level with the number of components in the above-described configuration, however, the cost for such production facilities will be substantial, which means that it is not practical.
There have been proposals for improving the machining accuracy of manufacturing a disc portion on which a recording medium is fixed, for making the assembly operations more efficient, or a proposal of new form of disc portion which is suitable for increasing the number of discs to be housed. However, there have been no proposals so far regarding a shape of disc portion that can withstand a shock when it is incorporated in a portable apparatus. There have been still other problems left to be solved; when the diameter of disc portion is reduced it turns out to be difficult to keep a stable levitation of head slider, and to maintain a sufficient strength at the outer circumference.
If a high recording density is to be implemented with a hard disc drive, it is essential to suppress the amount of levitation of a head slider to be approximately 20 nm or less. In reality, however, once a disc portion is deformed by an external impact, the amount of levitation easily go above the 20 nm. Under such a case, the GMR head, among others, which makes use of the effect of gigantic magnetic resistance suffers from a significantly deteriorated signal. Furthermore, the disc portion might get broken when a very great impact is given. So far, there has been no proposal addressing the above problems.
Regarding the ramp load mechanism, there have been several proposals; thinning the outer circumference of a disc portion for providing a ramp portion, providing a round protrusion belt as the ramp portion so that one end of a magnetic head ride thereon for a rest, making the outer circumference of a disc portion thicker for providing a ramp portion thereon, and so on. These structures, however, are accompanied by following problems. Since each disc is provided with a fitting hole at he center, there is an advantage that a disc drive can house a plurality of discs. However, the relationship between the bearing portion and the disc portion for mounting a recording medium remains the same as in the conventional hard disc drive. So, a deviation in the surface and a deviation in the outer circumference of the surface bring about big deviations in the surface and the concentricity, which naturally leads to big deviations in the surface and the concentricity of a ramp portion provided outside the recording region of recording medium. If the accuracy is deteriorated with the ramp portion, it turns out to be difficult for a head to perform a predetermined action of sheltering; the head unable to make the sheltering action might remain on the surface of recording medium.
Furthermore, in the conventional configuration a shaft is press-fit in a hub, and a round circular disc is attached on the hub. Therefore, the accuracy errors in the shaft length, the hub height, the disc thickness, and the processing accuracy in chassis and case cover, as well as the errors in the height of press-fit operation, accumulate. Which makes it difficult to raise the accuracy level, blocking the efforts for a higher recording density. A technical breakthrough is needed to implement a higher recording density.
The technologies so far proposed for suppressing influence of the magnetic fields escaping form rotating magnet of spindle motor on a recording medium are aimed to improve the influence during operation of a finished device. No consideration has been given on an adverse effect on a recording medium caused by the magnetic fields escaping from the motor or other magnetic members of a disc device during assembly. Now, consideration on the above aspect has become necessary in the face of an advanced technology under which the disc devices are being made still smaller and provided with an increasing recording capability; as a result, an influence of the escaping magnetic fields on a recording medium during manufacturing stage has been unable to disregard. However, no fundamental measure has been taken against the problems.
Technical points to be considered when taking measures against the influence of escaping magnetic fields on a recording medium during manufacturing stage include the following phenomena, for example;    (1) When gluing an already-magnetized rotating magnet with a recording medium having a magnetic layer thereon using a thermosetting adhesive, the magnetic layer is magnetized, although very slightly, by an integrated influence created by the magnetic field escaping from rotating magnet and the heat applied during the processing,    (2) In the finished state of a disc device, a rotating magnet is disposed opposed to the iron core of stator; so, redundant magnetic flux is converged to the core iron. It hardly escapes to the disc side. However, during the stage of rotor assembly when there is no stator core iron around, there exists a magnetic flux high enough to ill-affect a recording medium disposed in the information storage side.    (3) When a recording medium of hard disc drive is exposed to a substantial noise magnetic field while the magnetic layer is heated, noise signal is overlaid on the magnetic layer of recording medium in accordance with the noise magnetic field. If a servo signal is disturbed, the normal operation of recording/reproducing is impaired.